The universal mobile telecommunications system (UMTS) is a network technology allowing transmission both of voice and high-speed data. It is part of the third-generation (3G) wireless standards, as specified by the Third-generation partnership project (3GPP). Wideband code-division multiple access (WCDMA), also called wideband CDMA, is one method for radio transmission used in UMTS. UMTS is a development of GSM/GPRS supporting packetized voice and data transmissions.
The method called high speed downlink packet access (HSDPA) is an enhancement of UMTS for increasing the capability of transmitting data, resulting in a reduced cost per transmitted bit and a greater spectral efficiency, in addition to significant improvements in downlink data speeds. HSDPA can give improvements of at least two to three times the current capacity. It is based on the WCDMA standard and uses the same spectrum. HSDPA uses quadrature phase shift keying (QPSK) and 16-quadrature amplitude modulation (16QAM).
The HSDPA method uses a distributed architecture in order to achieve a low delay link adaptation by performing the most important process steps in the radio base stations (RBSs) and thus close to the air interface, see FIG. 1. HSDPA uses well established processing steps, including fast physical layer (L1) retransmission for faulty packets, combining and link adaptation techniques, to obtain improved packet data transmission.
The HSDPA process steps basically include:                scheduling in the radio base stations for the downlink packet data operation;        higher-order modulation;        adaptive modulation and coding;        hybrid automatic repeat requests (HARQs) for retransmissions;        physical layer feedback of the momentary channel condition; and        transmission in a high-speed downlink shared channel (HS-DSCH) allowing several users to share an air interface channel.        
In the following some important features of HSDPA will be described.
1. Adaptive modulation and coding
HSDPA uses advanced link adaptation and adaptive modulation and coding.
2. Fast scheduling
In HSDPA data traffic is scheduled in the radio base stations. HSDPA uses information on channel quality, terminal capabilities, quality of service (QoS), and power/code availability to achieve efficient scheduling of data packet transmissions.
3. Past L1 retransmissions
When a link error occurs, a mobile terminal immediately requests the retransmission of the lost or erroneous data packets. This operation is denoted as a method including hybrid automatic repeat requests (HARQs) for reducing the delays in and increasing the efficiency of retransmissions. HARQ control is performed in the radio base stations, as illustrated in FIG. 2.
4. Channel quality feedback
In the radio base stations, according to the HSDPA method, estimates of the channel quality of each active user are collected and used. This feedback provides current information on a wide range of channel variable physical layer conditions, including power control, ack/nack ratio, QoS, and HSDPA-specific user feedback.
5. High-speed downlink shared channels (HS-DSCHs)
HSDPA operation is carried in high-speed downlink shared channels using a frame length of only two milliseconds, compared to frame lengths of 10, 20, 40 or 80 ms used in previously used downlink shared channels (DSCHs). Such downlink shared channels are downlink transport channels, each of which may be shared by several user equipments. A downlink shared channel is used to carry dedicated control or traffic data from the SRNC (Serving Radio Network Controller). A DSCH will be associated with one or several downlink DCHs (Dedicated Channels). The HS-DSCHs provide 16-level quadrature amplitude modulation (16-QAM), link adaptation, and the combining of retransmissions in L1 with HARQs. HSDPA uses high-speed shared control channels (HS-SCCHs) to carry the required modulation and retransmission information. Uplink high-speed dedicated physical control channels (HS-DPCCHs) carry automatic repeat request (ARQ) acknowledgement messages, provide downlink quality feedback and transmit other necessary control information in the uplinks.
HSDPA requires a flow control algorithm or method that controls the transmission of data frames in an HS-DSCH, as specified by e.g. TS 25.401 from 3GPP, between a radio network controller and a radio base station. The algorithm for flow control is not standardized, but control messages, e.g. the message “Capacity Allocation”, are standardized. In order to manage the flow control, the RBS calculates the allocations to be carried in the “Capacity Allocation” messages sent to the RNC, and the RNC sends data frames in the HS-DSCH to the RBS according to the information in the “Capacity Allocation” messages, one allocation of capacity for each flow of data. When there is more data to send from the RNC, the information element (IE) “User Buffer Size” (UBS) in the HS-DSCH data frames is larger than zero. When the data frame has emptied the RNC buffer for the respective flow of data, UBS is set to zero.
The flow control algorithm has to manage limitations for both the air-interface and the Iub HS-DSCH bandwidth, Iub being the interface between an RNC and an RBS.
More particularly, the transfer of a data frame in an HS-DSCH from an RNC to an RBS is made in the following way. After the RNC has been granted capacity by the RBS, as obtained from a capacity allocation control frame or from an initial capacity allocation control frame received from the RBS, as described in 3GPP TS 25.433, and the RNC has data waiting to be sent, the data frame is used to transfer the data in the HS-DSCH. If the RNC has been granted capacity by the RBS using the initial capacity allocation control frame as described in 3GPP TS 25.433, this capacity is valid for only the first data frame transmission in the HS-DSCH. When data is waiting to be transferred, and a capacity allocation control frame has been received, a data frame in the HS-DSCH will be transmitted immediately according to the received allocation, i.e. using the bandwidth corresponding to this allocation. Each data frame sent in an HS-DSCH includes the information element “User Buffer Size” to indicate the amount of data pending for a respective flow for an indicated priority level.
In making the transmission of data frames from an RNC to an RBS efficient the Iub HS bandwidth limitations must be considered. When dedicated channels for e.g. voice is connected, the bandwidth for HS-DSCHs will be less, provided that, as will be assumed hereinafter, that these dedicated channels and HSDPA traffic share the same physical link. If the bandwidth for HS-DSCHs causes that HS-DSCH data frames are lost, it has a bad impact on TCP based applications. It is then better to lower the effective bitrate for HS-DSCHs to a level in which the data frame loss ratio becomes reasonable.